Three principal types of such seals are known. In the first type an internal or external contact ring is pressed against the surface to be sealed by a biasing member of square or rectangular cross-section and having a width which is substantially smaller than that of the contact ring. It is also known to provide seals having square or rectangular-section lateral guide sleeves flanking a triangular-section core which itself contacts the surface to be sealed without the intermediary of special sleeves. The third type of seal comprises an assembly having a central supple core which bears against the surface to be sealed and is maintained by two guide and protecting sleeves each having a nose or end whose inner face comes in contact with one of the inclined flanks of the central filler.
A principal disadvantage of such seals is that they are tight even when the pressure differential across them is relatively slight. This can lead to the so-called dieseling effect wherein the air oil-vapor mixture spontaneously explodes, thereby destroying the seal rapidly.